The present invention relates generally to a superheterodyne receiver and more particularly to utilizing a mixer as such a receiver.
FIG. 1 illustrates a conventional superheterodyne receiver. The typical superheterodyne receiver 10 includes a low noise amplifier (LNA) 12 which is placed near an antenna (not shown) for receiving the signal. The signal typically is at a level that is comparable to a noise component therein. The LNA 12 is coupled to an image-rejection filter 14. The image-rejection filter 14 is coupled to an active mixer 16.
FIG. 2 illustrates a conventional LNA 14. The low noise amplifier includes first and second transistors coupled to a current source. The gain stage is degraded from the Miller Effect capacitance. FIG. 3 illustrates a typical active mixer 16. The active mixer includes four switching devices coupled to the LNA to provide the differential output voltage. Typically the gain of mixer 16 is reduced and the noise is increased due to commutation of the switching devices that decreases the signal level and provides an image noise simultaneously. Each of the stages (LNA 12, filter 14 and mixer 16) provides both gain (G) and noise factor (F) the receiver 10.
In a typical receiver, the noise factor is strongly influenced by the gain distribution, i.e.,
FTotal=F1st+(F2ndxe2x88x921)/G1st+(F3rdxe2x88x921)/G1st/G2ndxe2x80x83xe2x80x83(1)
Where F1st is the LNA 12, F2nd is the filter 14, and F3rd represents the mixer 16. In general, the mixer 16 is a noisy stage and the filter 14 is required to reject image noise from entering the mixer 16. The filter 14 typically provides a loss to the receiver.
In a receiver, the sensitivity is determined by the ratio of the received signal and FTotal at the antenna. Since the received signal is dictated by the antenna design, so the designer is limited to noise reduction option in order to maximize the sensitivity. A common approach to minimize the noise factor of the receiver is to provide a LNA 12 with good noise performance and high gain to keep FTotal low.
In this configuration, the LNA 12 provides high gain to suppress the noise contributions from the filter 14 and the mixer 16. However, the problem is that with a conventional receiver there is always a compromise between noise and linearity. To further describe this problem, refer now to the following. In a typical receiver there are intermodulation distortion products (IDPs) which are produced by signals that are not of the same frequency but which create spurious signals within the frequency band.
In a typical receiver, the IDPs are produced by two strong signals at one and two times the frequency offset from the frequency of interest interacting with each other in a manner which provides intermodulation distortion at the exact frequency of interest in operation. For example, if the channel is 3 and there are two strong signals, one at channel 10 and one at channel 17, those two signals have a frequency spacing of 7. This frequency spacing of 7 can cause two spurious signals or intermodulation products at the same frequency range of the signal that is desired, i.e., channel 3 and 24. These spurious signals, or IDPs, then interfere with the signal that is desired. Accordingly, a figure of merit for the spurious signals is called IIP3, which stands for input third order intercept point.
IIP3, as is well known, describes how strong the IDPs are. The system IIP3 is degraded by the added gain stage that is meant to improve the system noise figure. For example, if an LNA whose gain is 15 dB is added in, the the IIP3 of the system will be degraded by no less than 15 dB. Accordingly, equation (2) describes the strength of the IDPs of the system.
1/IIP3Total=1/IIP31st+G1st/IIP32ndxe2x80x83xe2x80x83(2)
Accordingly, while added gain will improve system noise figure for better receiver sensitivity, the added gain could degrade the total IIP3 and therefore adversely affect the performance of the system.
In order to solve the dilemma, the traditional approach is to add the LNA 12 (gain stage 1), and improve the IIP3 of the mixer 16 (gain stage 2). In so doing, in an ideal environment an increase on the G1st (numerator) should be matched by an increase on the IIP32nd (denominator) to maintain good IIP3Total., as suggested by the second term in Eq.2. However, improving the IIP3 means either higher power consumption or more feedback (gain reduction) for the mixer. Since higher power consumption is not a preferred solution in general, the designer would end up with a situation that extra gain is added in the front but some gain has to be thrown out in the following stage, thus requires even more stages to be inserted in the system thereby adding to the inefficiency of the system.
Accordingly, what is needed is a system and method for overcoming the above-identified problem. The present invention addresses such a need.
A mixer is disclosed. The mixer comprises a high isolation gain stage and an impedance transformation network coupled to the gain stage. The mixer includes a plurality of switching devices coupled to the network and a phase shifter coupled to the plurality of switching devices. The mixer is utilized as a receiver and a low noise amplifier is not needed.
A receiver in accordance with the present invention achieves high gain and low noise in the mixer and therefore eliminates the need for a separate LNA. In so doing, an isolation gain stage achieves high gain, and image noise is rejected before entering the mixer stage.